spirit level

spirit level, tool for determining whether a surface is horizontal. It consists essentially of a slightly bent transparent tube that is held in a frame. The tube contains some alcohol, ether, or similar fluid but is not entirely filled, so that it also contains a small bubble. The position of the bubble within the tube indicates whether the instrument is horizontal. The spirit level used by carpenters and masons has two tubes at right angles to each other, so that the device registers a vertical as well as a horizontal position. The plumb line was used to determine vertical position from ancient times until the 19th cent. At that time the spirit level, which was invented in France during the 17th cent., came into general use and replaced the plumb line in some applications.